This invention relates generally to scanning imaging systems, and more particularly to the production of images by scanning imaging systems having a table that is capable of moving at a variable rate during a scan of an object or a person.
Known scanning computed tomography (CT) imaging systems do not allow for the pitch of a helical scan to vary during a scan. Instead, the helical pitch (i.e., table speed) is held constant during a CT scan. To achieve constant table speed, the table is positioned a distance away from a desired starting location and accelerated to a desired speed. During the acceleration, the x-ray beam is not turned on and no data is acquired. When the table reaches the desired speed and passes the starting location, the beam is turned on and data acquisition starts.
Faster and higher performance scanning CT imaging systems allow for increased clinical applications. Several new clinical applications in the cardiac and perfusion areas require that a scanning CT imaging system vary its pitch while scanning. For example, a typical head perfusion requires coverage of 4–8 cm along the patient's axis (i.e., a z-axis of the imaging system, normally corresponding to a head-to-toe axis of a patient). Known multi-slice CT imaging systems are able to cover only 2 cm at any given instant. Thus, it would be advantageous to acquire the perfusion data in a shuttle mode in which the patient table is moving back and forth during scanning to cover the entire perfusion organ. A shuttle mode such as this requires that data acquisition and reconstruction occur during the ramping-up and ramping-down periods of table motion. Known scanning CT imaging systems do not provide this capability.